Answer: First off, if your dad's a bear, what does that make you?
second: The common ground is like the example, grouchiness.
for number 19, the common ground is being good, which both the boy and the angle are. For number 20, the common ground is hot or bad breath.
To create new ones:
21. Jeff has a heart of gold. The common ground is goodness.
22. It's raining cats and dogs. The common ground is the loudness of both the animals and the storm outside.
23. Her soft voice is music to my ears. The common ground is that both are beautiful to listen to.